Databases are computerized information storage and retrieval systems. A Relational Database Management System (RDBMS) is a database management system (DBMS) which uses relational techniques for storing and retrieving data. Relational databases are organized into tables which consist of rows and columns of data. The rows are formally called tuples. A database will typically have many tables and each table will typically have multiple tuples and multiple columns. The tables are typically stored on direct access storage devices (DASD), such as magnetic or optical disk drives for semi-permanent storage.
A table is assigned to a tablespace. The tablespace contains one or more datasets. In this way, the data from a table is assigned to physical storage on DASD. Each tablespace is physically divided into equal units called pages. The size of the tablespace's pages is based on the page size of the buffer pool specified in the tablespace's creation statement. The bufferpool is an area of virtual storage that is used to store data temporarily. A tablespace can be partitioned, in which case a table may be divided among the tablespace's partitions, with each partition stored as a separate dataset. Partitions are typically used for very large tables.
RDBMS software using a Structured Query Language (SQL) interface is well known in the art. The SQL interface has evolved into a standard language for RDBMS software and has been adopted as such by both the American National Standards Institute (ANSI) and the International Standards Organization (ISO). The SQL interface allows users to formulate relational operations on the tables either interactively, in batch files, or embedded in host languages, such as C and COBOL. SQL allows the user to modify the data and as the data is being modified, all operations on the data are logged in a log file.
When a primary copy of a database is corrupted, for example, due to a system or media failure, or when a user wants to remove modifications to the data (i.e., back out the changes), the primary copy of the database is recovered by applying changes found in the log to a secondary copy of the database. That is, through use of the log file, the copied data is modified, based on the operations and data found in the log file.
Typically, for database systems, it is recommended that dual copies (duplex) of the log be maintained to prevent a single point-of-failure. The recommendation is to maintain the secondary copy of the log on a separate I/O path from the primary copy of the log. In the event of a failure reading the primary copy of the log, the secondary copy of the log is used to satisfy the request.
Normally, all log read requests are directed to the primary copy of the log, reserving the secondary copy of the log for error recovery. However, in the case where a system failure has occurred, but the primary copy of the log remains accessible, a number of recovery tasks often run in parallel. Accordingly, it often occurs that there are a number of substantially concurrent requests to the log. This action results in a queuing of the requests and a delay in the recovery action.
It is therefore an object of this invention to remove a system recovery bottleneck that is presented when too many requests to the log are queued.
It is another object of the invention to provide a method for more efficient use of the secondary copy of the log.